1. Field of the Invention
The present invention is directed to an optical coupling device for facilitating the coupling of an optical fiber to an optical waveguide and to a method for fabricating such coupling device. Another aspect of this invention relates to optical fiber to optical waveguide interconnect comprising the coupling device of this invention, and arrays comprising a plurality of such arrays.
2. Description of the Prior Act
Recent developments in the area of optical communications have provided a large number of optical waveguide devices for the control and routing of light. Optical waveguide devices which are created on independent substrates are often referred to as planar integrated optical devices or photonic devices. These devices can be further characterized as passive devices, those which serve only to route the propagation of light along a particular path, and active devices, those which control some function of the propagating light, such as its intensity or polarization, or which dynamically control the path along which the light propagates. However, the propagation of light on a substrate bearing an optical waveguide is usually suitable only for short propagation distances, usually much less than a meter. For longer distance propagation the optical fiber is the medium of choice due to its excellent transmission characteristics and ability to be fabricated in lengths of many kilometers. Therefore, if an optical waveguide device is to be utilized in an optical communication or sensor or distribution system, it is usually required that it be coupled to an optical fiber at least one point, and often at many points, on the substrate, An optical waveguide device which has been coupled to lengths of optical fibers for ease of insertion into an optical fiber system is sometimes referred to as a "pigtailed" waveguide device by those skilled in the art. Light propagates through the core region of optical fibers and these core regions can be as small as a few microns in diameter. Thus, in order that the fiber to waveguide coupling is accomplished in an efficient manner that does not waste most of the light, the alignment of the fiber to the waveguide is of necessity a critical parameter. Numerous articles and methods have been devised in the prior art to provide for efficient coupling of optical fibers to substrates bearing optical waveguide devices. The need for critical alignment tolerances has resulted in a high degree of complexity and cost for these devices and methods of the prior art. There are many descriptions of methods which utilize silicon "V-grooves" as a positioning element, such as U.S. Pat. No. 4,767,174, which make use of the fact that certain crystalline orientations of silicon can be preferentially etched to a high degree of accuracy. This is accomplished by a series of lithographic steps including resist coating and exposure, followed by liquid etching. However, the V-groove, once fabricated, serves only to position the optical fiber relative to the surface of the silicon wafer. It still remains to position the fiber end relative to the waveguide end. This is usually accomplished by micromanipulation of the two components relative to each other followed by fixing the alignment by an optical quality glue. Micromanipulation is an expensive and time consuming operation for use in a manufacturing operation. Alternatively, the V-groove and optical fiber can be positioned relative to the waveguide by the use of additional positioning elements, but these also increase the complexity and therefore cost of the method. Even when the V-groove technique is utilized only to couple two optical fibers to one another, as in U.S. Pat. No. 4,973,126 there are several additional positioning elements required. Also, the V-groove techniques serve to position an optical fiber relative to some surface, such as that of the silicon itself, but the V-groove does not provide any force to retain the optical fiber in position. That is, the optical fiber can easily slip out of the groove unless one or more additional elements are present to provide some retaining force. Typically, a cover plate or a second substrate containing V-grooves is forced down in contact with the optical fibers to hold them in the V-grooves and an optical cement or photopolymer is used to hold the assembly together.
Several methods have been taught in the prior art for creating optical fiber positioning devices on the substrate which also bears a waveguide or integrated optical device. These techniques do serve to reduce the overall number of separate elements which must be assembled to complete the optical fiber to waveguide coupling. However, the techniques still remain complex in manufacture or they lack sufficient retaining force to provide a simple, low-cost, yet effective method of providing such a coupling. U.S. Pat. No. 5,150,440 describes a method wherein waveguides are printed in a plastic film, in which film are subsequently created rectangular grooves, as by excimer laser ablation of the film near the waveguide terminus. Subsequently, the film bearing the waveguide and grooves is laminated on both sides with additional film layers such that the openings of square cross section are created in the plane of the waveguide. The ends of the openings are generally made accessible and smooth by a subsequent microtoming step. The square opening is then filled with a liquid photopolymer adhesive and optical fibers are then admitted into the square opening and fixed in place with a cementing process. This method involves a large number of manufacturing steps such as lamination and excimer laser ablation and is limited to the specific waveguide type of invention, that is, to waveguides created in thin plastic sheets of the same order thickness as the waveguide which can be conveniently notched by methods such as laser ablation. The method does not provide for a convenient method of attaching optical fibers to optical waveguides created on other useful substrates such as semiconductor wafers, polyimide circuit board materials, glass, lithium niobate and other crystalline and ceramic substrates. Neither does it provide for a method of attaching optical fibers to any film except those which are the thickness of the optical waveguide, unless they are supported on another layer which is removable after the notching process, which adds further complexity to the manufacturing process. U.S. Pat. No. 4,735,677 describes a method for providing guides for aligning optical fibers on the surface of a silicon substrate. In this method it is necessary to first grow a layer of glass on the silicon wafer by a soot process wherein a glass precursor is treated by flame hydrolysis to deposit glass particles on the silicon, followed by heating in an electric furnace to consolidate the glass. This layer of glass is then lithographically patterned and etched, as by reactive ion etching (RIE), to form the positioning elements. After formation of these elements, an optical fiber can be inserted between them and fixing is accomplished with an adhesive or by melting the glass with a CO.sub.2 laser beam. This technique involves a great number of processing steps and is limited to substrates which are not damaged by high temperature processes or those which do not contain sensitive electronic devices which would be damaged by an RIE etch. A number of desirable substrates for waveguide devices such as polyimide printed circuit boards and polycarbonate would not be useful for these reasons. Further, like the V-groove techniques, it serves only to position the optical fibers yet provides no rigidity or retaining force to the coupling except through the addition of an adhesive or another high temperature melting process. Several methods for forming fiber optic positioning devices in molded thermoplastics are provided. Japan Kokai Patent 278004 provides for optical fiber guide grooves of approximately triangular cross section by a method of compression molding a thermoplastic resin in a die. This essentially provides a die-molded plastic version of a silicon V-groove. While eliminating the need for a lithographic etch to produce the V-grooves, the remaining drawbacks of the technique remain. Thus the need to align the V-grooves to the waveguide is still present and the need for additional elements to hold the fiber optics into the V-grooves still exists and is taught in Kokai Patent 278004. Japan Patent Publication 254404 further teaches hybrid optical circuits formed by die-molding an inorganic glass or a plastic resin. The result is a substrate bearing positioning elements for optical devices and optical fibers such as with U.S. Pat. No. 4,735,677, but without the need to deposit a soot layer and etch. However, the shape of the optical fiber holding channel must be such that the dimension of the top is equal to or larger than the bottom, or it will be difficult or impossible to release the substrate from the die mold. Japan Patent Publication 254404 teaches that release from the mold is made easier if the holding blocks are made V-shape or trapezoid shape with the top of the channel larger than the bottom of the channel. Thus again, this method partially solves the problem of positioning an optical fiber relative to a waveguide, but does not provide for a secure retaining force on the fiber. In all of these prior art methods, some force must be applied to hold the optical fiber within the positioning grooves. Further, if a liquid optical cement or photoactive monomer or optical matching fluid is introduced to the coupling region it will wet between the optical fiber and the wall of the positioning groove by the force of capillary action. This will serve to float the fiber within the groove and thus misalign it with the waveguide, unless the secondary element is present to hold the optical fiber down within the groove with a greater force. The methods of die molding a plastic resin suffer the additional drawback of the necessity of maintaining the die free of any contamination. If the grooves in the master die which will form the waveguide or the positioning elements become contaminated on repeated use, the error will be faithfully repeated on each successive molding, and prevent accurate optical fiber positioning. Finally, in the die mold method of Japan Patent Publication No. 254404, the substrate, waveguide and positioning elements are formed together from the same material. This precludes the ability to position optical fibers to waveguides created on other desirable substrates such as silicon wafers, lithium niobate wafers and printed circuit boards, and limits the choice of waveguide material to the class of die-moldable plastics and glasses.
Thus, the need remains for devices and methods which allow for simple, accurate positioning of optical fibers to optical waveguides on a variety of desirable substrates, which do not require many additional manufacturing steps or positioning elements or elaborate alignment by micromanipulator and which provide a secure retaining force to the optical fiber without the need for additional elements held in place by optical adhesives or thermal heat treatments.